Amplitude weighting in holography

ABSTRACT

Holographic system in which the signal (object) beam, the reference beam, or the readout beam is weighted (i.e., shaped) in amplitude as a function of the spatial coordinates of the hologram plane. Either the reference beam or the signal beam may be weighted in accordance with the light distribution of the signal beam to provide smaller variation in modulation, or unity modulation, for the various spatial frequencies in the signal beam over the entire area of the hologram. The readout beam may be shaped or weighted to further compensate for the variation in diffraction efficiency caused by the interaction of the residual variations in the modulation level and the nonlinearities of the recording medium.

[451 Mar. 28, 1972 [54] AMPLITUDE WEIGHTING IN HOLOGRAPHY [5 l 1 Int.Cl. ..G02b 27/22 [58] Field oiSearch ..350/3.5, 162 SF [56] ReferencesCited UNITED STATES PATENTS 9/l97l Phelps et al ..350/3.5

OTHER PUBLICATIONS Lohmann et al., 25A Physics Letters 570- 571(10/1967) Stroke et al., 7 Japanese Journal of Applied Physics 764- 766(7/1968) Murata et al., 7 Japanese Journal of Applied Physics 30l- 302(3/1968) Pennington. II IBM Tech. Disc. Bulletin 322 (8/1968) PrimaryExaminer-David Schonberg Assistant Examiner-Robert L. ShermanAttorney-Donald R. Greene [5 7] ABSTRACT Holographic system in which thesignal (object) beam, the reference beam, or the readout beam isweighted (i.e., shaped) in amplitude as a function of the spatialcoordinates of the hologram plane. Either the reference beam or thesignal beam may be weighted in accordance with the light distribution ofthe signal beam to provide smaller variation in modulation, or unitymodulation, for the various spatial frequencies in the signal beam overthe entire area of the hologram. The readout beam may be shaped orweighted to further compensate for the variation in diffractionefficiency caused by the interaction of the residual variations in themodulation level and the nonlinearities of the recording medium.

12 Claims, 3 Drawing Figures AMPLITUDE WEIGHTING IN IIOLOGRAPHY Thepresent invention relates to holography and. more particularly, toholograms constructed near or in the Fourier transform plane.

It is an object of the present invention to provide a new and improvedmethod and apparatus for constructing a hologram, particularly a Fouriertransform, or near Fourier transform hologram, in which the signal beamand reference beam have approximately the same amplitude distribution inthe recording plane.

A further object of the present invention is to provide a new andimproved holographic method which reduces degradation in thereconstructed beam on reconstruction because of factors such asnon-linearity of the recording medium and the wide range of amplitudesusually present for the various spatial frequencies in the signal beam.

Another object of the present invention is to compensate for the effectof non-linearities in the hologram, such as nonlinearity in the responseof the recording medium and insufficient dynamic range, by weighting theamplitude of the readout beam.

Yet another object of the present invention is to provide a new andimproved method and apparatus for constructing a hologram, particularlya Fourier hologram, in which the quality of the recording is improved byweighting the signal or the reference beam approximately in accordancewith the amplitude of the spatial frequencies of the signal beam.

A further object of the present invention is to provide a new andimproved method and apparatus for making a holographic recording,particularly a Fourier transform recording, which lessens the normalvariation in modulation of the reference beam for different spatialfrequencies.

Further objects and advantages of the present invention will be apparentfrom the following detailed description of the present invention madewith reference to theaccompanying drawing forming a part of the presentspecification for all subject matter disclosed therein and in which:

FIG. 1 is a diagrammatic showing of a specific holographic recordingsystem for practicing the preferred embodiment of the present invention;

FIG. 2 is a diagram showing the modulus of a signal spectrum in theFourier transform plane as a function of the spatial frequency; and

FIG. 3 is a diagrammatic showing of another specific holographicrecording system for practicing the present invention.

Referring to FIG. 1, a laser I provides a light beam 12 which is splitinto two beams l4, 15 by a beam splitter 18. The beam 14 passes througha focusing lens and a pinhole plate 24 located in the focal plane of thelens 20. The beam diverges from the pinhole in the plate 24 and passesthrough a collimating lens 26 which forms a collimated light beam 28which then illuminates an object, a transparency 31 in the illustratedembodiment, to form an object beam or signal beam 32 containing theinformation to be holographically recorded. A diffuser or self imaginggrating may optionally be employed between lens 26 and transparency 31.A lens in the object beam images the object with magnification at animage plate 36 As is well known in the art, there distribution in thefocused signal fonn of the light from the object in or near this planeis termed a Fourier transform hologram. For purposes of simplification,the term Fourier transform hologram is used in this application toindicate holograms made in the Fourier transform plane or sufficientlynear the plane to essentially maintain the characteristics of a Fouriertransform hologram.

In accordance with the preferred embodiment of the present invention, aholographic recording medium 33 is placed in the Fourier transform planeand a collirnated reference beam 40 is caused to interfere with thesignal beam to establish interference fringes which are recorded on therecording medium. The recording medium is exposed through an apertureplate 34.

is a plane in which the light beam is the Fourier transplane and arecording made a relative distribution of light spatial frequencysignals. As illustrated in FIG. frequencies in a typical 2, theamplitude of the low spatial Fourier transform is considerably greaterthan the amplitude of the higher spatial frequencies.

In the embodiment of the present invention, illustrated in FIG. I, thereference beam is shaped to provide a light distribution at therecording medium which approximately corresponds to the lightdistribution of the signal beam to provide approximately unitymodulation of the reference beam by the signal beam. As illustrated inFIG. 1, a mask 44 is placed in the reference beam. The transmittancecharacteristics of the mask is also a function of the x and ycoordinates of the mask and is such that the light distribution for thereference beam at the recording medium approximately corresponds to thatof the signal beam.

The mask 44 may be made by exposing photographic film to a Fouriertransform signal beam from atypical object without interference with areference beam. The film is then developed, if it is notself-developing, to provide a mask having transmittance characteristicswhich correspond to the light distribution of the signal beam at therecording medium. With the mask in position in the collimated referencebeam, the amplitude of the reference beam as a function of thecoordinates at the recording medium will correspond to the signal beam.A neutral density filter may be positioned in the beam 15 from the beamsplitter to adjust the peak magnitude of the reference beam to provideunity modulation at the recording medium for the various spatialfrequencies in the signal beam.

When photographic film is used as a transmission mask, the grain of thefilm introduces higher spatial frequencies which are a noise factor inthe reference beam. Preferably, a spatial filter is used to filter thesefrequencies from the reference beam. To accomplish this, a focusing lens48 is placed in the reference beam with the mask 44 located in the frontfocal plane of the lens. A spatial filter 49 is placed in the Fouriertransform plane to block the higher spatial frequencies from the mask 44so that the reference beam at the recording medium essentially dependsonly on the light amplitude transmitted by the mask. A collimating lens50 is disposed between the filter and the recording medium. Lenses 48and 50 may also be used to image the mask 44 at the holographicrecording medium 33 with unity magnification.

In making the mask 44, the object can be rotated during exposure so thatthe magnitude of the reference beam at the recording medium hasrotational symmetry.

It is to be recognized that while it is desirable to provide unitymodulation for all spatial frequencies, in practice this is difficult toaccomplish, but the described techniques provide a much smallervariation in modulation and will cause more spatial frequencies tocontribute to the imaging on reconstruction. The contribution of morespatial frequencies and improved diffraction efficiency because ofgreater modulation and smaller variation in modulation for the spatialfrequencies results in an improved reconstructed image over conventionalpractice even though there is a variation in modulation for the variousspatial frequencies.

In FIG. 3, a photochromic material or other self-developing material isutilized to provide a mask 44' in the reference beam, which maskperforms the same function as the mask 44 of FIG. 1. The parts of FIG. 3which correspond to elements shown in FIG. 1 have been given the samereference designation and a detailed description of these elements willnot be repeated except where it clarifies the description of theelements or their functions difi'er from those of FIG. I.

in the system illustrated in FIG. 3, the object 31 is located a distancefrom the lens greater than the focal length of the lens and is imaged atan image plane 51. The recording is made in a near Fourier transformplane 52.

A beam splitter 53 is located in the signal beam between the object andthe lens 30' and a portion of the beam is directed through an imaginglens 54 which corresponds in characteristics to the lens 30 and whichimages the object as in the case of lens 30'. The lens 54, the object,and the mask 44' are relatively located so that the photochromicmaterial is disposed in the near Fourier transform plane for light fromthe object passing through the transforming lens 54. The plane in whichthe photochromic material is positioned corresponds to the plane 52 forthe lens 30 so that the light distribution on the photochromic materialand, in turn, the transmittance characteristics of the photochromicmaterial, corresponds to the light distribution of the signal beam atthe recording medi um. As is known in the art, the light transmittancecharacteristics of photochromic material can be made to vary directlywith the amplitude of the light illuminating the material.

The present invention also contemplates the weighting of the readoutbeam to compensate for residual variations in modulation and for othernonlinear characteristics. Weighting of the readout beam may also beused to produce more faithful reconstructions for holograms made in aconventional manner. For example, the non-weighted reference beam onrecording may have a magnitude as indicated by the dashed line 60 inFIG. 2. The non-linearity of the recording medium in this case mayresult in saturation and a loss of the amplitude information for the lowspatial frequencies. In accordance with the present invention, a maskmay be constructed for weighting the readout beam in accordance with thecharacteristics introduced by the nonlinear characteristics of therecording. As will be appreciated by one skilled in the art, thenecessary transmittance characteristics for the mask weighting thereadout beam may be determined empirically to provide a higher amplitudereadout beam in those areas of the hologram wherein non-linearity orsaturation of the recording medium, or loss of diffracting efficiencywould normally cause a degradation of the light amplitude for that areaof the hologram.

The present invention recognizes that the signal beam may be weighted toprovide smaller variation in the modulation for the spatial frequenciesof the signal beam. In this case, a mask having transmittancecharacteristics which vary inversely with the amplitude of the spatialfrequencies will be placed in the signal beam adjacent the recordingmedium. The mask is used in the Fourier transform plane of the objectand this plane is imaged by a lens at a recording plane where -theoff-axis reference beam interferes with the signal beam and the hologramis recorded. Preferably the reference beam is adjusted in amplitude toprovide approximately unity modulation. in such a system a mask withinverse transmittance characteristics relative to the first mask is usedin the readout beam to preserve the relative amplitudes for the variousspatial frequencies.

While the reference beam is a collimated beam in the illustratedembodiment, it will be recognized that other types of beams could beused and a lens system provided which images the mask on the recordingmedium. This would also be true in the situation where the readout beamis weighted.

What is claimed is:

1. In a method of constructing a hologram of an object on a recordingmedium located at or near the Fourier transform plane as theinterference pattern between a signal beam of light obtained byillumination of the object and a reference beam of light arriving offaxis relative to the signal beam, the step of amplitude weighting thelight distribution in one of said signal beam and said reference beamwith an approximation of the Fourier transform amplitude distribution ofsaid object to suppress the effect on the interference pattern at therecording medium of large variations in amplitude of the lightdistribution in the signal beam primarily attributable to gross detailfeatures of said object. and thereby tend to produce approximately unitylight modulation of said interference pattern throughout the hologram.

2. The method according to claim 1, wherein said gross detail featuresof said object include contrast in lightness and darkness betweenvarious portionsof said object.

3. The method according to claim 1, wherein said recording medium isself developing upon exposure to said interference pattern.

4. The method according to claim l, wherein said object is atransparency containing data to be holographically recorded.

5. The method according to claim 1, wherein said amplitude weighting iscarried out by shaping the amplitude of the light distribution in thesignal beam prior to interference with said reference beam, by placing amask having a light transmittance characteristic which producesconsiderable blockage of the larger amplitude variations, in the path ofthe signal beam between the object and the recording medium.

6. The method according to claim 1, wherein said amplitude weighting iscarried out by shaping the amplitude of the light distribution in thereference beam to more nearly match the amplitude of the lightdistribution in the signal beam.

7. The method according to claim 6, wherein said shaping is performed byplacing a mask having a light transmittance characteristic possessingsaid match in the path of said reference beam prior to interference withsaid signal beam.

8. in a method of reconstructing an image of an object from a hologramthereof recorded on a recording medium as an interference patternbetween a signal beam of light obtained by illumination of the objectand a reference beam of light incident on the recording medium off axisrelative to the signal beam, wherein one of said signal beam and saidreference beam were subjected to amplitude weighting of the lightdistribution thereon to suppress the effect on the interference patternof large variations in amplitude of the light distribution pattern inthe signal beam primarily attributable to gross detail features of theobject, and wherein said reconstructing the image is performed byilluminating the hologram with a readout beam of light, the step ofamplitude weighting the light distribution in the readout beam tosubstantially restore the effect of said large variations in amplitudeof the light distribution in said signal beam to the light diffracted bythe hologram, and thereby to restore said gross detail features of theobject in the image thereof.

9. in a system for constructing a hologram of an object on a recordingmedium as the interference pattern between a signal beam of lightobtained by illumination of the object and a reference beam of lightincident on the recording medium located at or near the Fouriertransform plane off axis relative to the signal beam, the combinationcomprising means for generating said signal beam and said referencebeam,

means for directing the signal beam and the reference beam onto therecording medium in interfering relationship, and

means for amplitude weighting the light distribution in one of saidsignal beam and said reference beam prior to interference therebetweenat said recording medium to suppress the effect on the interferencepattern of large variations in amplitude of the light distribution inthe signal beam primarily attributable to gross detail features of saidobject, and thereby tend to produce approximately unity light modulationof said interference pattern throughout the hologram.

10. The system according to claim 9, wherein 12. The system according toclaim 10, wherein said mask is configured for amplitude weighting thelight distribution in said reference beam by passing light in saidreference beam in a distribution pattern approximating the lightdistribution in said signal beam.

# i l i i

1. In a method of constructing a hologram of an object on a recordingmedium located at or near the Fourier transform plane as theinterference pattern between a signal beam of light obtained byillumination of the object and a reference beam of light arriving offaxis relative to the signal beam, the step of amplitude weighting thelight distribution in one of said signal beam and said reference beamwith an approximation of the Fourier transform amplitude distribution ofsaid object to suppress the effect on the interference pattern at therecording medium of large variations in amplitude of the lightdistribution in the signal beam primarily attributable to gross detailfeatures of said object, and thereby tend to produce approximately unitylight modulation of said interference pattern throughout the hologram.2. The method according to claim 1, wherein said gross detail featuresof said object include contrast in lightness and darkness betweenvarious portions of said object.
 3. The method according to claim 1,wherein said recording medium is self developing upon exposure to saidinterference pattern.
 4. The method according to claim 1, wherein saidobject is a transparency containing data to be holographically recorded.5. The method according to claim 1, wherein said amplitude weighting iscarried out by shaping the amplitude of the light distribution in thesignal beam prior to interference with said reference beam, by placing amask having a light transmittance characteristic which producesconsiderable blockage of the larger amplitude variations, in the path ofthe signal beam between the object and the recording medium.
 6. Themethod according to claim 1, wherein said amplitude weighting is carriedout by shaping the amplitude of the light distribution in the referencebeam to more nearly match the amplitude of the light distribution in thesignal beam.
 7. The method according to claim 6, wherein said shaping isperformed by placing a mask having a light transmittance characteristicpossessing said match in the path of said reference beam prior tointerference with said signal beam.
 8. In a method of reconstructing animage of an object from a hologram thereof recorded on a recordingmedium as an interference pattern between a signal beam of lightobtained by illumination of the object and a reference beam of lightincident on the recording medium off axis relative to the signal beam,wherein one of said signaL beam and said reference beam were subjectedto amplitude weighting of the light distribution thereon to suppress theeffect on the interference pattern of large variations in amplitude ofthe light distribution pattern in the signal beam primarily attributableto gross detail features of the object, and wherein said reconstructingthe image is performed by illuminating the hologram with a readout beamof light, the step of amplitude weighting the light distribution in thereadout beam to substantially restore the effect of said largevariations in amplitude of the light distribution in said signal beam tothe light diffracted by the hologram, and thereby to restore said grossdetail features of the object in the image thereof.
 9. In a system forconstructing a hologram of an object on a recording medium as theinterference pattern between a signal beam of light obtained byillumination of the object and a reference beam of light incident on therecording medium located at or near the Fourier transform plane off axisrelative to the signal beam, the combination comprising means forgenerating said signal beam and said reference beam, means for directingthe signal beam and the reference beam onto the recording medium ininterfering relationship, and means for amplitude weighting the lightdistribution in one of said signal beam and said reference beam prior tointerference therebetween at said recording medium to suppress theeffect on the interference pattern of large variations in amplitude ofthe light distribution in the signal beam primarily attributable togross detail features of said object, and thereby tend to produceapproximately unity light modulation of said interference patternthroughout the hologram.
 10. The system according to claim 9, whereinsaid means for amplitude weighting is a mask with preselected lighttransmissive characteristics.
 11. The system according to claim 10,wherein said mask is configured for amplitude weighting the lightdistribution in said signal beam by considerably blocking the passage oflight in said signal beam at said large variation portions thereof. 12.The system according to claim 10, wherein said mask is configured foramplitude weighting the light distribution in said reference beam bypassing light in said reference beam in a distribution patternapproximating the light distribution in said signal beam.